In general, an electric vehicle is comprised of a battery for supplying electric power, a PWM inverter for converting DC power of that battery into AC power of variable voltage and variable frequency, a 3 phase AC motor, a speed sensor installed on that 3 phase AC motor, a drive system comprised of several current sensors for detecting current that flows through the winding of that 3 phase AC motor, a torque reference operation means that determines torque reference of the 3 phase AC motor based on accelerator opening, an AC current reference operation means that determines the size and phase of current flowing through the winding of the 3 phase AC motor based on that torque reference and the rotating angular speed, a current control means that determines AC voltage reference so that the 3 phase AC current will follow that AC voltage reference, and a PWM signal generation means that determines the gate signal applied to the PWM inverter based on that AC voltage reference.
Generally, the configuration of a torque control system for the electric vehicle is based on the torque reference which is dispatched from the driver via the accelerator opening, electric current which is detected from the electric current sensor and the rotating angular speed which is detected by the speed sensor. For this reason, as disclosed in Japanese Patent Application Laid Open No. 3-277101, safety during motion was secured by introducing a double sensor system so that one can switch to the other sensor in the event one sensor fails, thereby driving the motor based on information obtained from this auxiliary sensor.
As shown above, while reliability of sensing itself is improved by introducing a multiple sensor system, reliability of the control system as a whole is not necessarily improved owing to increase in cables and connector terminals for taking in the detected information.
Further, there are cases where other sensors in a multiple sensor system that are supposed to serve as the back-up also fail depending on the cause of failure. In these cases, there was a problem of possible danger that occurred when switching to an auxiliary means.